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discovery rests with Owen and with Professor Poult on, who
pointed out in 1884,1 that the ovum of OrnitTiorTiyncJius is very
lareje as compared with those of other Mammalia (6 mm. as
against *2 mm..)* that it is filled with yolk, and that it completely
fills the follicle, being surrounded by two layers of follicular
cells only. This latter fact was proved by CaldwelL Subse-
quently Gyldberg 2 and I 3 described the ovarian ovum of Echidna,

FIG.  46.—Ovarian egg of MchicLna.    b, Basilar membrane ; fet follicular epithelium j
o, oil globules ; vm, vitelline membrane ; y1, y2, yolk.    (Partly after Caldwoll.)
showing it to be identical with that of Ornitfoorfiynchus* ]Later
still a more elaborate and beautifully illustrated £>aper was pub-
lished by Caldwell4 upon the early stages of development in the
Monotremata and Marsupials, in which the ovum of the former
was accurately described (see Fig. 46). In the particulars
mentioned above, the ovum of the Monotremata is practically
identical with that of the large-yolked ova of the Sanropsida.
1  Quart. Jowm. Micr. Set, xxiv. 1884, p. 9.
2  Sf.S. Jen. Gesells* 188S, p. 1.
8 Proc. Moy. Phys. Soc. j&din. Tiii. 1885, p. 854,
' * PMfc. Trtms. clxxviii. 1887, p. 463.